System and method for the production of alpha type gypsum using heat recovery

ABSTRACT

The present invention relates to a system and associated method for the production of gypsum in manufacturing plant. More specifically, the invention relates to the production of alpha-type gypsum in a gypsum board manufacturing plant. The system yields increased efficiencies by capturing heat given off during processing steps and using that heat to reduce the energy needed for calcination. The invention finds particular application in the production alpha-type gypsum. The present invention is described in greater detail hereinafter in conjunction with the following specific embodiments.

RELATED APPLICATION DATA

This is a continuation of provisional application Ser. No. 60/758,790filed on Jan. 13, 2006 and entitled “Process for Production of AlphaPlaster at Board Plants Using Heat Recovery from the Board Plant Stack.”The contents of this co-pending application are fully incorporatedherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a system and method for the production ofgypsum in a plant. More particularly, the present invention relates torecovering heat at various locations within a board manufacturing plantand using the recovered heat to reduce the energy needed for subsequentcalcination.

2. Description of the Background Art

It is known in the art to calcine gypsum in the manufacture of plasters,stuccos, gypsum wallboards and other building materials. However, theplants that produce gypsum-based building materials tend to be extremelyinefficient users of energy as large amounts of energy are needed toadequately dry gypsum. For instance, some processes require 60 lbs. ofwater to be evaporated for every 100 lbs of gypsum produced. Thebackground art contains numerous examples of attempts to increase theefficiency of gypsum manufacturing methods.

Some of these systems recover the heat from calcination and use it forfurther calcination. For example, U.S. Pat. No. 2,934,830 to Zvejnieksdiscloses a method whereby hot water vapors obtained by calcinatinggypsum are used to further calcinate and dry the gypsum. To achievethis, the apparatus includes a drying shaft that transports dried gypsumupwardly for removal at an upper outlet. Hot water vapors from acalcination apparatus are directed and delivered downwardly through theshaft as the gypsum is concurrently delivered upwardly. The hot watervapors are used to both heat and dry the gypsum within the shaft.

Furthermore, U.S. Pat. No. 1,798,857 to Tyler discloses a method for thecontinuous calcination of gypsum. The method employs steam obtained fromgypsum that is calcined in kettles to heat a steam jacket and chamber.The steam provided to jacket serves to calcinate gypsum within chamber.Additional hot gases collected within a separator can also be reused.

Lastly, U.S. Pat. No. 1,746,294 to Tyler discloses a method for thecontinuous calcination of gypsum. The method employs a separatingchamber that includes a steam dome and a condenser, whereby excess steamcan be collected, condensed and stored for subsequent use. The apparatusfurther includes a pre-heater wherein gypsum is partially calcined.Heated steam from the partial calcination is collected and used forfurther calcination within an additional calcinating chamber.

Although the above referenced inventions achieve their respectiveobjectives, none address the inefficiencies unique to the manufacture ofalpha-type gypsum, nor do they address the inefficiencies inherent inthe manufacture of gypsum based building materials, such as gypsumboard. The present invention is directed to overcoming theseinefficiencies.

SUMMARY OF THE INVENTION

It is therefore one of the objects of the present invention tomanufacture alpha-type gypsum in an energy efficient manner.

Another objective of the present invention is to remedy theinefficiencies inherent in the production of gypsum-based buildingmaterials.

It is also an object of this invention to provide a process for theproduction of gypsum building materials wherein heat generated from thedrying of gypsum-based building materials is recovered and used forcalcination.

Still another object of this invention is to provide a process whereinheat can be recovered at multiple locations throughout a buildingmaterial plant and used in calcination.

The foregoing has outlined rather broadly the more pertinent andimportant features of the present invention in order that the detaileddescription of the invention that follows may be better understood sothat the present contribution to the art can be more fully appreciated.Additional features of the invention will be described hereinafter whichform the subject of the claims of the invention. It should beappreciated by those skilled in the art that the conception and thespecific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of the primary embodiment of the gypsumproduction method of the present invention.

FIG. 2 is a schematic diagram of an alternative embodiment of the gypsumproduction method of the present invention.

FIG. 3 is a schematic diagram of an alternative embodiment of the gypsumproduction method of the present invention.

Similar reference characters refer to similar parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a system and associated method for theproduction of gypsum in a manufacturing plant. More specifically, theinvention relates to the production of alpha-type gypsum in a gypsumboard manufacturing plant. The system yields increased efficiencies bycapturing heat given off during processing steps and using that heat toreduce the energy needed for calcination. The invention finds particularapplication in the production alpha-type gypsum. The present inventionis described in greater detail hereinafter in conjunction with thefollowing specific embodiments.

The primary embodiment of the present invention is described inconjunction with FIG. 1. FIG. 1 is a schematic illustrating the variouscomponents within a plant 20 that is designed for the manufacture ofgypsum-based building materials. Although the particular plantillustrated is utilized in the production of gypsum wallboard, thepresent invention can be carried out in connection with the manufactureof a variety of building materials.

Plant 20 includes a board dryer 22 for transporting and drying a seriesof gypsum boards. Dryer 22 includes vertically arranged dryer stacks 24for use in evacuating gasses from inside the dryer 22. A condenser 26 isassociated with one or more of these stacks and in used in capturing thelatent and sensible heat of the steam leaving the stacks in a mannermore fully described hereinafter. The system further includes a mixingvessel 28 and stirrer 32 that are used to collect and mix the componentsnecessary to form a gypsum slurry. A Moino-type or positive displacementtype pump 34 is utilized to deliver the slurry from mixing vessel 28into an autoclave 38. Autoclave 38 is pressurized and includes amultiple mixer blade 40 and steam injector, whereby the slurry is heatedunder a pressure of 3-4 Bar and converted to an alpha hemihydrate andthereafter transported to an exit 42. A flow meter and pressure let downvalve 43 control the exit flow. Once processed in autoclave 38, thealpha gypsum can be used in the formation of various buildingcomponents, including gypsum board.

The alternative embodiments of FIGS. 2 and 3 illustrate further aspectsof the system. These alternative embodiments generally include a valve44 for controlling the flow of the processed gypsum into a flash tank 46wherein the gypsum is cooled and excess steam is collected. The excesssteam is then routed to a condensate scrubber 56 via steam conduit 58.Steam within scrubber 56 is then condensed and the condensate isreturned to vessel 28. Gypsum from flash tank 46 is then routed to ahydroclone separator 48 where unwanted particulates are removed.Additional filtration can be carried via a belt filter 52. The operationof the system, in both its primary and alternative embodiments, isdescribed in more detail hereinafter.

With reference to FIG. 1, the construction of board dryer 22 is wellknown to those skilled in the art of manufacturing gypsum board. Typicaldryer construction includes one or more conveyer belts for transportingboards from an inlet to an outlet. The inlet can be associated with oneor more additional conveyors for use in carrying out additionalmanufacturing steps. For example, the boards may be cut to a desiredsize or shape and thereafter transported to the inlet of dryer 22.Likewise, additional conveyors can be associated with the outlet ofdryer 22 for downstream processing of the boards. This additionaldownstream processing may include, for example, board coating and/orpackaging steps.

Heating within dryer 22 can be accomplished via direct injection fromgas burners. The heat from these burners can be diluted withrecirculated gas so that the temperature within the dryer will be withinthe approximate range of 240° F. to 380° F., depending upon the zone.The internal temperature of dryer 22 can be maintained by insulatingdryer 22 using known techniques. Dryer 22 preferably includes a seriesof stages that allow for the progressive drying of the boards. Theembodiment depicted in FIG. 1 includes three such stages 22(a), 22(b),22(d). Each of the dryer stages includes a corresponding chimney orstack 24(a), 24(b) and 24(c). The stages may likewise include separateheaters (not shown).

By way of this successive heating, the boards passing through dryerstages 22(a), 22(b) and 22(c) are heated to a degree sufficient toevaporate any excess moisture within the boards. Stacks 24(a), 24(b) and24(c) permit this excess moisture to be removed from dryer 22 as steam.In a multi-zone dryer, such as the one depicted, most of the moisturewill be removed in the final zone 22(c), and most of the steam will exitdryer 22 through the final stack 22(c). Once the boards exit dryer 22they are cooled and transported for additional processing.

Steam leaving stacks 24(a), 24(b) and 24(c) will have a temperature ofanywhere between 240° F. and 380° F., and more often this steam will bewithin the range of 300° F. and 380° F. In accordance with the presentinvention, both the latent and sensible heat from the steam is capturedvia a condenser 26 and used to calcinate raw gypsum into an alpha-typegypsum. Although a condenser can be associated with each of the stacks(24(a), 24(b), and 24(c)) of dryer 22, the preferred embodiment utilizesjust a single condenser that is operatively connected to primary stack24(c). This design is a reflection of the fact that most of the vapor,and thereby most of the heat, will escape from dryer 22 via the primarystack 24(c).

Condenser 26 can employ any of a number of different and widely knownconstructions and can be, for example, either an air cooled or liquidcooled condenser. The construction includes a heat exchanger (for usewith an air cooled condenser) or coils (for use with a liquid cooledcondenser), either of which are thermodynamically coupled to stack 24.With a liquid cooled condenser, a cooling medium, such as water or anyof a number of commonly known refrigerants, is circulated through thecoils via a pump (not shown). By bringing the cooling medium intothermodynamic contact with the steam, the overall temperature of theexiting steam is reduced. By reducing the temperature of the steam atleast a portion of the steam is converted from the vapor phase to theliquid phase. This liquid phase is a hot condensate with a temperaturewithin the approximate range of 180° F. to 200° F., although thetemperature of the condensate can be as high as 210° F. Thesetemperatures are provided only as an example, as the exact temperatureof the condensate will depend upon a number of factors, such as thetemperature and quantity of steam leaving the stack, the construction ofthe condenser and the type of working fluid utilized. Condensate fromcondenser 26 is then collected and routed via pipes to mixing vessel 28.

With continuing reference to FIG. 1, mixing vessel 28 is a collectionpoint for raw gypsum, moisture and condensate from condenser 26. Astirrer 32 is included in order to sufficiently blend these componentswithin the vessel 28. In the preferred embodiment, there is a 50:50ratio by weight between the moisturized gypsum, on the one hand, and thecondensate on the other. It has been found that this weight ratioproduces a slurry with a consistency that is beneficial for furtherprocessing. Additionally, as a result of the increased temperature ofthe condensate that is added to tank 28 (e.g. between 180° F. to 210°F.), the resulting slurry mixture has a temperature within theapproximate range of 140° F. to 160° F. Again, however, thesetemperature ranges are provided as a representative example as the exactranges will depend upon a variety of factors. The heated slurry is thentransported to autoclave 38 for further processing.

Autoclave 38 can be any of a number of standard autoclave designs thatare commonly used in wet or slurry type calcination. Autoclave 38 is asealed vessel that includes stirrers 40 and a steam inlet. The internalpressure and temperature of autoclave 38 are increased via steamsparging. By heating raw gypsum within autoclave 38, typically to atemperature of about 280° F., calcination is carried out in accordancewith the following equation: CaSo₄·2H₂O+heat=(CaSo₄ ½H₂O)+1.5H₂O. Thus,as is well known in the art of gypsum processing, calcination convertscalcium sulfate dihydrate into a calcium sulfate hemi-hydrate.Furthermore, carrying out the calcination under pressure yields analpha-type calcium sulfate hemi-hydrate. Alpha-type plaster has a highmechanical strength with a compact crystal structure and low waterdemands. Thus, use of alpha-type plaster is sometimes preferred forbuilding materials because it yields hard, low porosity casts.

By first heating the slurry in mixing tank 28, autoclave 38 only needsto raise the slurry temperature from the starting temperature of 140° F.to 160° F. to the calcinating temperature of 240° F. By contrast, usingan unheated slurry would require heating the slurry within autoclave 38from a starting temperature of approximately 60° F. to the calcinatingtemperature of 240° F. This is the energy savings realized byrecapturing the latent and sensible heat from dryer 22. The processedalpha-type gypsum is then removed from autoclave 38, via exit 42 andvalve 43, for further processing, such as with a hydroclone separator 48or belt filter 52 (note FIGS. 2 and 3). Thereafter, the processed gypsumcan be incorporated into the gypsum based building materials, such aswallboards, using known techniques.

In still yet further embodiments of the present invention, described inconjunction with FIGS. 2 and 3, additional heat recovery is utilized tofurther increase the temperature of the slurry delivered to autoclave38. This additional heat recovery generates an even greater energysavings during calcination. In the embodiment of FIG. 2, the processedgypsum from autoclave 38 is delivered, via pressure let down valve 44,to a flash tank 46. Flash tank 46 permits the alpha gypsum slurry tocool and depressurize. Steam generated by the gypsum within flash tank46 is collected and routed to condensate scrubber 56 via conduit 58. Thecollected steam within scrubber 56 is then condensed via slurry frommixing tank 28. More specifically, slurry from tank 28 is routed toscrubber 56 via a sprayer where it is used as a working fluid to furthercool the captured steam and bring it into the liquid phase as acondensate. The result is a hot condensate that is delivered back tomixing vessel 28 via conduit 54. The additional heated condensate fromscrubber 56 serves to increase the temperature of the slurry evenfurther to an approximate range of 170° F. to 190° F. Again, increasingthe temperature of the slurry in this fashion yields substantial energysavings during subsequent calcination within autoclave 38. The alphagypsum slurry within tank 46 is then removed via a pump for furtherprocessing by the hydroclone separator 48 and belt filter 52. All othercomponents are the same as the primary embodiment of FIG. 1.

It has been found that the energy required for taking the gypsum slurryfrom its normal temperature of 60° F. to the starting point of 180° F.represents two-thirds of the energy required to take the slurry from 60°F. to 240° F. As a result, a substantial energy savings is realized bythis additional heat recovery.

A still further energy savings is realized by even additional heatrecovery. The alternative embodiment of FIG. 3 illustrates a systemwherein a portion of the gypsum slurry within tank 28 is routed tocondenser 26 via a valve and conduit 62. This portion of the gypsumslurry is then utilized as the cooling medium in condenser 26, wherebythe slurry is used to cool the steam leaving stack 24(c). The remainingslurry is removed from tank 28 via a valve for delivery to autoclave 38for processing as described above. All other components are the same asthe alternative embodiment of FIG. 2. This embodiment results in an evengreater energy savings as the slurry is used to bring the vapor into theliquid phase. This embodiment also eliminates the need for anindependent cooling medium.

The present disclosure includes that contained in the appended claims,as well as that of the foregoing description. Although this inventionhas been described in its preferred form with a certain degree ofparticularity, it is understood that the present disclosure of thepreferred form has been made only by way of example and that numerouschanges in the details of construction and the combination andarrangement of parts may be resorted to without departing from thespirit and scope of the invention.

Now that the invention has been described,

1. A system for the production of alpha type gypsum in board plantswherein the gypsum is produced using heat recovery, the systemcomprising: a board dryer including a conveyor and a heating elementwhereby a number of gypsum boards are transported and heated to therebyremove excess moisture from the gypsum boards, the board dryer includingone or more stacks to permit excess moisture from the gypsum boards tobe removed from the board dryer as steam; a condenser associated withone of the stacks of the board dryer, the condenser using a coolingmedium to cool the steam within the stack to produce a condensate with atemperature within the approximate range of 170° F. to 190° F.; a mixingtank wherein condensate from the condenser is mixed with a combinationof raw gypsum and moisture in a 50-50 ratio by weight to produce agypsum slurry with a temperature within the approximate range of 140° F.to 160° F.; a pump for delivering a portion of the gypsum slurry fromthe mixing tank to the condenser wherein the gypsum slurry is used asthe cooling medium to produce condensate; an autoclave for processingthe gypsum slurry from the mixing tank, the autoclave pressurizing andheating the gypsum slurry whereby the gypsum slurry is calcined to yieldan alpha-type gypsum and steam, the alpha-type gypsum being used in theproduction of gypsum boards and wherein the steam generated from theproduction of the alpha-type gypsum is delivered to the mixing tank tofurther increase the temperature of the slurry to the approximate rangeof 170° F. to 180° F.
 2. A system for the production of alpha typegypsum in board plants wherein the gypsum is produced using heatrecovery, the system comprising: a board dryer including a conveyor anda heating element whereby a number of gypsum boards are transported andheated to thereby remove excess moisture from the gypsum boards, theboard dryer including one or more stacks to permit excess moisture fromthe gypsum boards to be removed from the board dryer as steam; acondenser associated with one of the stacks of the board dryer, thecondenser using a cooling medium to cool the steam within the stack toproduce a heated condensate; a mixing tank wherein heated condensatefrom the condenser is mixed with raw gypsum to produce a heated gypsumslurry; an autoclave for processing the heated gypsum slurry from themixing tank, the autoclave pressurizing and heating the gypsum slurrywhereby the gypsum slurry is calcined to yield gypsum and steam, thegypsum being used in the production of gypsum boards, wherein an energysavings is realized by using a heated gypsum slurry within theautoclave.
 3. The system as described in claim 2 further comprising apump for delivering a portion of the gypsum slurry from the mixing tankto the condenser wherein the gypsum slurry is used as the cooling mediumto produce condensate.
 4. The system as described in claim 2 whereinsteam vented from the production of the alpha-type gypsum is deliveredto the mixing tank to further increase the temperature of the slurry. 5.The system as described in claim 2 wherein calcination within theautoclave yields an alpha-type gypsum.
 6. The system as described inclaim 2 wherein adding heated condensate to the mixing tank produces agypsum slurry having a temperature within the approximate range of 140°F. to 160° F.
 7. The system as described in claim 2 wherein moisturizedgypsum and heated condensate are supplied to the mixing tank in a 50-50ratio by weight.
 8. A method for the production of alpha type gypsum ina plant comprising: drying gypsum based building materials to removeexcess moisture and collecting the excess moisture as steam; condensingthe collected steam so as to produce a heated condensate; mixing theheated condensate with raw gypsum to produce a heated gypsum slurry;calcinating the heated gypsum slurry to yield alpha gypsum and steam. 9.The method as described in claim 8 including the further step ofcondensing the steam generated from the calcination step to produce aheated condensate and mixing the heated condensate with the raw gypsumto further increase the temperature of the gypsum slurry.